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(see Standby Power in this section).
The RAT GCU functions independently from all other airplane power because it
is not on the same common network as the other generators.
(RAT deployment and operation are further described in Chapter 13, Hydraulics.)
Main Battery
The main battery provides standby power (until RAT deployment) to the
following:
• hot battery bus
• captains instrument bus
• first officers instrument bus.
Electrical Load Management
The electrical system is designed for automatic operation with manual overrides,
provided by flight deck controls.
The Bus Power Control Units (BPCU), in conjunction with the Generator Control
Units (GCU), provide information on the health status, coordinate load
management, power transfers, and fault clearing for the EPGSS.
Bus Power Control Units (BPCU)
The BPCUs act as the system gateway for data bus communications, through a
dedicated computer network.
In the event of a BPCU failure, a standby BPCU assumes all functions. These
BPCUs provide overall load management, protection, flight deck control /
indication, and power transfer functions. This ensures that the maximum number
of electrical loads are powered without exceeding electrical source capacities.
Modes of Operation
By using controllers to selectively power individual loads on various buses, the
airplane can power desired groups of loads together as an electrical operating
mode. The BPCU controls these modes, and issues commands to the network
which distributes power to the loads.
787 Flight Crew Operations Manual
Electrical -
System Description
6.20.14 Boeing Proprietary
These modes are defined as follows:
• Ground Service – when forward external power is connected, and the
GROUND SERVICE switch is selected to ON. In this mode, the 235 Vac,
115 Vac, 28 Vdc, and captains / first officers instrument buses are
powered. Only a selected group of loads are powered.
• Ground Handling – when forward external power is connected, and no
other power source powering buses. In this mode, the 235 Vac, 115 Vac,
28 Vdc, and captains / first officers instrument buses are powered. Only a
selected group of loads are powered.
• On-ground Battery Only – When the BATTERY switch is selected to ON
while the airplane is on the ground, and there are no other power sources.
The main battery relay closes to power the captains and first officers
instrument buses.
• In-air Battery Only – If all electrical power is lost, the captains / first
officers buses are powered by the main battery. In this mode, all of the
standby loads are powered (same as after the RAT is deployed), with the
exception of center pitot heat power. This mode is only applicable to the
captains / first officers instrument buses, hot battery bus, and the APU hot
battery bus.
• In-Air RAT Operation Only – This mode powers all loads connected to
the captains / first officers instrument buses, backup bus, and the APU hot
battery bus (see Standby Power in this section). Power is provided for
center pitot heat from the backup bus.
• Towing Power – This mode is entered by selecting the TOWING POWER
switch ON, while the BATTERY switch is OFF. The TOWING POWER
switch allows the hot battery bus and APU hot battery bus to power only
those loads necessary for towing operations (including night towing).
This excludes all other loads powered when the BATTERY switch is
selected ON, and provides extended towing times.
The loads that are powered by the TOWING POWER switch are:
• captains audio control panel / flight interphone
• flight deck dome lights
• wing and tail position lights
• brakes.
Generator Control Units (GCU)
There are six Generator Control Units (GCU), one for each generator installed on
the main engines, and one for each generator installed on the APU. Each GCU
functions independently from the airplanes power buses by receiving dedicated
power from its associated Permanent Magnet Generator (PMG). The GCU also
receives backup power from the airplane 28 Vdc buses. This allows for bus control
and protection during airplane dispatch with an inoperative generator, and to
prevent undesirable failure conditions.
787 Flight Crew Operations Manual
Electrical -
System Description
Boeing Proprietary 6.20.15
Each GCU provides the following functions for its associated generator source
and corresponding bus:
• power regulation, transfers, and fault protection
• Generator Control Breaker (GCB) signaling
• Bus Tie Breaker (BTB) signaling
　
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